The present invention relates to a method of ionising a sample and an ionisation source. The preferred embodiment relates to a method of ionising an eluent emitted from a liquid chromatography source.
It is known to provide an Electrospray ionisation (“ESI”) ion source which ionises eluent emitted from a Liquid Chromatography (“LC”) source. The Electrospray ionisation ion source comprises a capillary tube which is maintained at a constant high voltage during the course of a liquid chromatography separation in order to cause eluent emerging from the liquid chromatography source to be ionised by the high voltage applied to the capillary tube.
Reverse phase liquid chromatography is well known and it is common to mix two solvents (A,B) which are then passed through a chromatography column. It is conventional to use water as the first solvent (A) and an organic solvent such as acetonitrile, methanol or propanol as the second solvent (B).
During the course of a liquid chromatography experiment it is known to progressively vary the ratio of the two solvents A,B whilst keeping the total flow rate constant. Varying the ratio of the two solvents A,B causes different species of analytes to elute from the chromatography column at different times. For example, it is known to progressively increase the percentage of the second organic solvent B from a few percent to e.g. 95-100% over the course of a few minutes.
It will be apparent, therefore, that during the course of a conventional liquid chromatography separation the chemical composition of the mobile phase will vary as the percentage of organic solvent is increased with time.
According to known methods the chemical composition of the mobile phase may change over a time period of e.g. 2+ minutes in the case of Ultra Performance Liquid Chromatography (“UPLC”) (RTM) or over a longer time period of e.g. 7+ minutes for High Performance Liquid Chromatography (“HPLC”).
Whilst the chemical composition of the mobile phase is being varied, the capillary voltage is maintained constant during an entire run per instrument polarity mode.
A problem with the known arrangement and method of operation is that the sensitivity can be compromised particularly if compounds or analytes of interest elute across the entire liquid chromatography gradient.
US2006/0118713 (Matsui) discloses a liquid chromatography/mass spectrometry apparatus comprising a liquid chromatography column coupled to an Electrospray ionisation source. Prior to analysis of a sample, a preparatory analysis step is carried out to investigate the relationship between solvent mixture ratio and optimal applied voltage. The results from the preparatory analysis are stored as a voltage data curve. When performing an analysis of a sample, the solvent mixture ratio is varied as a function of time. Each time the solvent mixture ratio is varied, the voltage data curve is used to determine an optimal voltage value to be applied to the ionisation probe.
It is desired to provide an improved ion source for a mass spectrometer and an improved method of ionising an eluent emitted from a liquid chromatography source.